1. Field of Invention
The invention relates to an ink ejection apparatus that ejects ink droplets from a nozzle by driving an actuator to generate a pressure wave in an ink chamber, particularly to an ink ejection apparatus capable of ejecting three or more ink droplets for one printing command.
2. Description of Related Art
Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink jet type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on demand type that ejects ink droplets only used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.
A conventional ink ejection apparatus used in a drop-on demand type printing device includes a nozzle from which ink is ejected, an ink chamber that is provided on the back of the nozzle and stores ink, an actuator that changes the volume of the ink chamber, and a driving device that drives the actuator to generate pressure wave vibrations in the ink chamber causing ink to be ejected from the nozzle. This kind of ink ejection apparatus is of a design wherein the driving device drives the actuator to generate the pressure wave vibrations in the ink chamber in response to a change in the volume of the ink chamber, thereby ejecting ink from the nozzle.
The actuator may be made of a piezoelectric element that deforms through the application of a drive voltage. In this case, ink is ejected by applying a pulse voltage (hereinafter referred to as a drive pulse) to the piezoelectric elements from a drive circuit. In this kind of ink ejection apparatus, it is conceivable that the drive pulse is repeatedly applied to the actuator in response to one print command, to eject multiple ink droplets from one nozzle, so that one dot is formed. As one dot is produced from large quantity of ink in this case, an image can be formed having a deep color.
There is a shear mode type of piezoelectric element in an ink ejection apparatus using the piezoelectric element as the actuator, for example. An exemplary ink ejection apparatus of this kind, which also is the apparatus to which the invention is applied, is shown in FIGS. 10A and 10B. FIG. 10A is a sectional view taken along line 10xe2x80x9410 of FIG. 10B. FIG. 10B is a sectional view taken along line 11xe2x80x9411 of FIG. 10A.
As shown in FIG. 10A, an ink ejection apparatus 600 includes a bottom wall 601, a top wall 602, and elongated shear mode actuator walls 603 sandwiched therebetween. Each actuator wall 603 includes an upper wall 605 of piezoelectric material, which is adhesively attached to the top wall 602 and polarized in a direction indicated by an arrow 609, and a lower wall 607 of piezoelectric material, which is adhesively attached to the bottom wall 601 and polarized in a direction indicated by an arrow 611. Alternating pairs of actuator walls 603 form in alternation between ink chambers 613 and spaces 615, the spaces 615 narrower than the ink chambers 613.
As shown in FIG. 10B, a nozzle plate 617 having nozzles 618 is fixedly secured to one end of each ink chamber 613 and an ink supply source (not shown) is connected to the other end of each ink chamber 613 via a manifold 626. The manifold 626 includes a front wall 627 formed with openings in positions corresponding to the ink chambers 613, a rear wall 628 for sealing the space between the bottom wall 601 and the top wall 602. The manifold 626 is structured to distribute the ink supplied from the ink supply source to the front wall 627 and the rear wall 628 into each of the ink chambers 613.
Electrodes 619, 621 are provided on both sides of each of the actuator walls 603. Specifically, the electrode 619 is provided on the actuator wall 603 in the ink chamber 613 and the electrode 621 is provided on the actuator wall 603 in the space 615. The electrode 621 is also provided on the outer side surface of each of the two outermost actuator walls 603. The electrode 619 is covered by an insulating layer (not shown) to insulate it from the ink. Each electrode 621 is connected to a ground 623. Each electrode 619 provided in the ink chamber 613 is connected to a control unit 625 and carries a voltage (drive signal) described later.
When the control unit 625 applies the voltage to the electrodes 619 in the ink chambers 613, pairs of the actuator walls 603 deform in the shear mode such that the volume of each ink chamber 613 increases. An example of this operation is shown in FIG. 11. When a voltage of E volts, which is the crest value, is applied to an electrode 619c of the ink chamber 613c, an electric field develops in each of the actuator walls 603e and 603f in the directions indicated by the arrows 631 and 632, respectively. The actuator walls 603e and 603f deform in the shear mode to increase the volume of the ink chamber 613c. At this time, the pressure in the ink chamber 613c including the nozzle 618c decreases.
The voltage of E volts is applied to the electrode 619 only for a one-way propagation time T. While the voltage is applied, ink is supplied from the ink supply source. The one-way propagation time T is a time required for a pressure wave in the ink chamber 613 to propagate once in the lengthwise direction of the ink chamber 613. The one-way propagation time T is calculated by the following expression:
T=L/a,
wherein L is the length of the ink chamber 613 and a is the speed of sound in the ink in the ink chamber 613.
According to the theory of pressure wave propagation, the pressure in the ink chamber 613 reverses into a positive pressure when the one-way propagation time T passes after the application of the voltage. When the pressure becomes positive, the control unit 625 returns the voltage applied to the electrode 619 of the ink chamber 613 to zero volts, so that the deformed actuator walls 603e and 603f revert to their initial shape, as shown in FIG. 10A, and pressure is applied to the ink. The pressure reverted to positive and the pressure generated when the deformed actuator walls 603e and 613f return to their initial shape are combined into a relatively high pressure that develops near the nozzle 618c in the ink chamber 613c, ejecting ink from the nozzle 618c. 
However, when three or more ink droplets are ejected for one printing command in a drive waveform, as shown in FIG. 8E, the drive pulses are set as follows:
T1=T2=T3=T,
W1=W2=2T,
wherein T is the one-way propagation time, T1 is a pulse width of a drive pulse P1 for ejecting a first ink droplet, T2 is a pulse width of a drive pulse P2 for ejecting a second ink droplet, T3 is a pulse width of a drive pulse P3 for ejecting a third ink droplet, W1 is an interval between the drive pulses P1 and P2, and W2 is an interval between the drive pulses P2 and P3.
In this case, the application of the pressure to the ink chamber and the cancellation of the pressure application are performed in synchronization with the one-way propagation time T. In other words, the pressure is applied in accordance with a rising point of the ink pressure wave and the application of the pressure is cancelled in accordance with a falling point of the ink pressure wave. Therefore, the pressure wave is gradually amplified to perform efficient ink ejection. However, the pressure applied to the ink becomes greater whenever the ink droplet is ejected, and ejecting speed becomes faster for a later ink droplet. As a result of the influence of the pressure wave, the ink may be ejected from an adjacent nozzle, ink ejection may become unstable and the interval to eject ink droplets may become short when the printing command is continuously executed on the same nozzle. As shown in FIG. 9B, ink droplets 99 may coalesce into one along the trajectory. If the ink droplets 99 coalesce or unify, during the trajectory in this manner, deviation in trajectory occurs, lowering printing quality. Further, when the temperature of the ink is changed, the one-way propagation time T is also changed, becoming out of synch with the application of the ink pressure wave and the cancellation of the application. As a result, ink droplets vary in size, printing density is changed, and ink ejection becomes unstable.
The invention provides an ink ejection apparatus capable of ejecting three or more ink droplets for one printing command stably without dispersion in density over a wide range of temperatures and of preventing ink droplets from coalescing into a globule during the trajectory without difficulty to improve printing quality.
According to one aspect of the invention, an ink ejection apparatus includes a nozzle from which ink is ejected, an ink chamber provided on a back of the nozzle where the ink is stored, an actuator that changes a volume of the ink chamber, and a drive device that drives the actuator by applying a drive signal including a plurality of pulses to the actuator to cause the actuator to generate a pressure wave vibration in the ink chamber, thereby ejecting the ink from the nozzle. The drive device generates positive and negative pressure waves in the ink chamber through application of one drive pulse to the actuator. When three or more ink droplets are ejected for one printing command, the drive signal satisfies the following expressions:
0.8Txe2x89xa6T1xe2x89xa61.2T, 0.4Txe2x89xa6T2xe2x89xa61.2T, 0.4Txe2x89xa6T3xe2x89xa60.8T, W1 greater than W2, W1 greater than 2T,
wherein T1 is an effective pulse width of a drive pulse P1 to eject a first ink droplet, T2 is an effective pulse width of a drive pulse P2 to eject a second ink droplet, T3 is an effective pulse width of a drive pulse P3 to eject a third ink droplet, W1 is an interval between the drive pulses P1 and P2, W2 is an interval between the drive pulses P2 and P3, and T is a one-way propagation speed where a pressure wave is propagated in the ink chamber once.
Under these expressions, as T3 is set shorter than T and W1 is set longer than 2T, the first ink droplet ejection does not have an adverse effect upon the second and third ink droplets, thereby reducing the pressure applied to the ink during the ejection of the second and third ink droplets. This enables ink droplets to be ejected stably and separately thereby preventing the ink droplets from coalescing into one globule. The nozzle is not affected by the previous ink ejection and ink ejection by an adjacent nozzle, thereby improving printing quality. As there is no need to insert a non-ejection pulse between the drive pulses, as has been conventional, the invention can preferably correspond to high-speed printing. The ink droplets can be stably ejected over a wide range of temperatures, thereby stably obtaining a specific printing density.
In the above structure, it is preferable that T2, T3, W1, and W2 further satisfy the following expressions:
0.4Txe2x89xa6T2=T3xe2x89xa60.8T, 1.8Txe2x89xa6W2xe2x89xa62.2T, and 2.2Txe2x89xa6W1xe2x89xa62.8T.
It has been found from various experiments that printing quality can be improved further preferably and stably when T2, T3, W1 and W2 satisfy the above expressions.
Further, it is preferable that T1, T2 and T3 satisfy the following expression:
T1xe2x89xa7T2 greater than T3.
It has been found from various experiments that ink droplets can be further preferably ejected over a wide range of temperatures and a specific printing density can be stably obtained when T1, T2 and T3 satisfy the above expression.